Userspace can pass zero length segments via writev/readv

Userspace can pass zero length segments via writev/readv

Userspace can trigger an assertion by passing a zero-length segment
when assertions are enabled:

[27961.614792] VERIFY3(skip < iov->iov_len) failed (0 < 0)
[27961.614795] PANIC at zfs_uio.c:187:uio_prefaultpages()
[27961.614805] Call Trace:
[27961.614811]   dump_stack+0x45/0x57
[27961.614830]   spl_dumpstack+0x44/0x50 [spl]
[27961.614834]   spl_panic+0xbb/0x100 [spl]
[27961.614908]   uio_prefaultpages+0x134/0x140 [zcommon]
[27961.614930]   zfs_write+0x1fd/0xe80 [zfs]
[27961.615014]   zpl_write_common_iovec+0x7f/0x110 [zfs]
[27961.615035]   zpl_iter_write+0xa0/0xd0 [zfs]
[27961.615037]   do_iter_readv_writev+0x59/0x80
[27961.615063]   do_readv_writev+0x11b/0x260
[27961.615098]   vfs_writev+0x39/0x50
[27961.615100]   SyS_writev+0x4a/0xe0
[27961.615103]   system_call_fastpath+0x16/0x6e

The solution is to delete the assertion. This could potentially
occur in uiomove as well, which contains analogous assertions
that appear similarly unnecessary, so we remove those as well.

Reported-by: Jonathan Vasquez <jvasquez1011@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Issue #3792